In most alumina refineries, bauxite is digested in a caustic solution under conditions of elevated temperature and pressure. This yields a slurry of mud in a concentrated sodium aluminate solution which must then be clarified to produce a solids-free liquor and a thickened mud that is subsequently washed and discarded. The prevalent technique for slurry clarification involves allowing the solids to settle in gravity thickeners (or mud settlers), and decantation of the clarified liquor. The separation of the mud from the concentrated liquor is assisted with flocculants, whilst the “green” (or pregnant) liquor, which is free of all but the finest suspended solids, overflows from the mud settlers. It is normal for the decanted liquor to then be further clarified by filtration, typically using pressure filters. This so-called “security” or “polishing” filtration step is critical in ensuring that the pregnant liquor is free of suspended mud particles that would otherwise result in contamination of the product alumina.
Unaided, the cloths employed in these filters would blind very quickly. This occurs because the fine suspended solids in the green liquor become entrapped within the weave of the cloth, and then proceed to form a dense, highly resistive bed at the filter's surface.
To prevent this, it is common practice to supplement the feed to the polishing filter with a filter aid, which acts to prevent cloth blinding by the continuous formation of a bed of solids which trap the mud particles whilst still allowing the free flow of liquor through the interstices of the bed. An ideal filter aid will be cheap, chemically inert, and of such a size that the channels that form between the filter aid particles are just small enough to trap the mud particles, but not so small that they restrict the flow of liquid, or contribute to blinding of the filter cloth themselves. In most alumina refineries, this role is performed by tricalcium aluminate (also referred to as TCA, C3A or C3AH6).
TCA is chosen as it meets the above requirements tolerably well. It is cheap and relatively simple to produce, being formed through the reaction of lime (either as quicklime or, more usually, slaked lime) with caustic aluminate solutions. Generally, the production of TCA is performed in concentrated liquors (either pregnant or spent) at temperatures of approximately 100° C., in tanks dedicated to the purpose. The initial products of the reaction are calcium aluminate species of the C4A type (the most common form of which is also referred to as hydrocalumite). These intermediate calcium aluminate species are thermodynamically unstable under such conditions and possess relatively high solubility products. Use of these intermediate species as filter aids is uncommon, because their comparatively high solubility can result in calcium contamination of the product alumina. TCA, on the other hand, has an extremely low solubility, so its use does not result in appreciable contamination of the refinery's liquor stream. For this reason, sufficient residence time is allowed in the reaction vessel for the initial products of the reaction to “age” before use, forming relatively pure particles of the thermodynamically stable tricalcium aluminate (TCA). The tanks in which the filter aid is produced are therefore often referred to as “lime ageing” tanks
A serious drawback of the prior art technique for TCA filter aid production is that the distribution of TCA particle sizes is often very wide, and there is invariably a high proportion of very fine particles. This results in poor filtration rates and low filter cloth life, and necessitates the use of a large number of filters to achieve adequate filtrate flow.
Given the comparatively low solubility of calcium in Bayer liquors, it is reasonable to assume that reactions between calcium and the aluminate ion occur at the particle surface, rather than via the dissolution of calcium hydroxide and subsequent re-crystallisation of calcium aluminate. Examination of the reaction of individual slaked lime particles in Bayer liquors as a function of time, using scanning electron microscopy and XRD analysis suggests that this is indeed the case. A surface layer of C4A-type material forms rapidly at the surface of the particle, while the core remains unreacted. The surface develops a characteristic reticulated appearance arising from the formation of many randomly oriented platelets of C4A. At this early stage, the particle still retains the general form and size of the parent lime particle.
As the reaction proceeds, the core of unreacted lime diminishes, suggesting either that calcium ions are diffusing outwards towards the surface, or that aluminate ions are diffusing inwards. In addition, some of the surface C4A crystals begin to recrystallise into the familiar octahedral TCA crystals. However, C4A and TCA have a substantially different density to calcium hydroxide and as the reaction proceeds, internal stresses are generated within the crystal. Cracks and fissures develop within the particle, and as the reaction proceeds, the structure begins to crumble. Given sufficient time, the particle will ultimately degrade into individual TCA crystals, each only a few microns in size.
The inventors believe that it is this mechanism that results in the severely skewed, overly fine size distribution that is characteristic of TCA filter aid produced using the prior art technique. Given this mechanism, it is therefore not surprising that attempts to improve filter aid morphology and size distribution by altering the conditions in the lime ageing tanks are not successful.
In the absence of any effective means to improve the size distribution of the TCA filter aid itself, a few processes have been published which seek to alleviate the effect of poor quality filter aid. These prior art techniques invariably utilise flocculants, which serve to bind the decanted mud particles and fine TCA crystals into larger floccules. For example, in U.S. Pat. No. 5,091,159 Connelly et al. describes the addition of dextran to the thickener overflow and filter aid to improve filtration performance. A similar approach for use in refineries where sand filters are employed, rather than filter presses, is described in U.S. Pat. No. 5,716,530 by Strominger et al. Barham et al1 describes the results of tests in which the filter aid is supplemented with a flocculant after it has been formed in lime ageing. However, these approaches are highly dependent upon the quality of the filter aid and the amount of suspended mud, and results can be variable. Furthermore, the flocculant itself can seriously impair filtration, particularly if overdosed (as can occur readily in an attempt to control an excursion in either TCA particle size or suspended mud).
The terms ‘A’ and ‘C’ used throughout this Specification refer to the alumina and caustic concentrations of a Bayer liquor, as per conventional alumina industry parlance. Hence ‘A’ is the concentration of sodium aluminate, expressed as the equivalent concentration of Al2O3, in g/L. The ‘C’ concentration is the sum of the sodium aluminate and sodium hydroxide concentrations, expressed as the equivalent concentration of sodium carbonate, in g/L.